Rapid exhumation since at least 13 Ma in the Himalaya recorded by detrital apatite fission-track dating of Bengal fan (IODP Expedition 354) and modern Himalayan river sediments

Apatite fission-track analysis of middle Bengal fan sediments (IODP expedition 354) and modern Himalayan river sediments shows that most of the detrital apatites are very young compared to their depositional ages, independent of their uranium content. Bengal fan apatites display an average central age lag time as short as Myr since at least ∼13 Ma. Such lag times reflect a mean exhumation rate on the order of at least 1-3 km/Myr. The occurrence of detrital apatites with relatively short AFT lag times since at least 13 Ma indicates that there have always been areas of rapid erosional exhumation, supplying detrital apatites to the fluvial system and delivering them to the paleo-Ganges and/or –Brahmaputra plains and finally to the Bengal fan. It also supports that temporary storage of detrital apatites in the floodplains or delta has always been negligible since at least 13 Ma. Comparison of the AFT data of the Bengal fan with those of the Central and Eastern proximal Neogene Himalayan foreland basin shows that both paleo-Ganga and –Brahmaputra catchments provided apatites with similar short lag time to the distal Bengal Fan basin. In the modern drainage system of the Bengal fan, the apatites with young fission-track cooling ages are principally derived from areas where the topography has a sharp relief controlled by threshold hillslope processes and stream power resulting in landslide erosion as a coupled response to tectonic and fluvial forcing. By analogy with the modern erosion processes in the Himalayan range, we suggest that over the past 13 Ma, apatites were mainly derived from areas of sharp relief, where river stream power was high and hill slopes close to the threshold angle. As the exhumation signal is rather consistent since the late Miocene the detrital apatite fission-track data are either not sensitive enough to detect rapid climatically controlled changes in exhumation rates, or overall long-term erosion rates on the orogen scale are not strongly affected by climatic variations such as the variability of the Indian Summer Monsoon. Given the already rapid exhumation rates controlled by tectonics, the impact of climate variability on surface erosion rates cannot be detected with our data, especially in the case of erosion processes dominated by threshold hillslope model.